1. Field of the Invention
This invention relates generally to semiconductor devices and, in particular, is directed to a semiconductor device, for example, a hot electron transistor.
2. Description of the Prior Art
Recently, in accordance with a remarkable increase of information processing, it is strongly requested to realize a logic element which can operate at higher speed. In the prior art, such a high speed logic element has been realized so far by fine-processing and highly or largely integrating a silicon device.
At the same time, a high speed logic element has been earnestly developed by using a compound semiconductor having a high electron mobility such as GaAs and so on. However, the realization of the high speed logic element by using a two dimensional fine processed silicon device arrives at its limits and a rapid progress for realizing such a high speed logic element by this method can no longer be expected.
On the other hand, as a ultra-high speed logic element, various kinds of transistors have been proposed, which can operate on the basis of different operation principles from those of an ordinary field effect transistor (FET) and a bipolar transistor, and a hot electron transistor (hereinafter simply referred to as HET) is typically enumerated as one of such transistors.
FIG. 1 is a cross-sectional view illustrating the fundamental structure of an example of the prior art HET. As shown in FIG. 1, the HET is formed of, for example, a high-doped n-type GaAs emitter layer 1, an intrinsic AlGaAs emitter barrier layer 2, n-type GaAs base layer 3, an intrinsic AlGaAs collector barrier layer 4 and a high-doped n-type GaAs collector layer 5. In FIG. 1, reference letters E, B and C designate an emitter terminal, a base terminal and a collector terminal of the HET, respectively.
In this HET, the emitter terminal E is grounded and the collector terminal C is applied with a voltage +Vcc. When a predetermined on-state voltage V.sub.BE is applied between the emitter terminal E and the base terminal B, a majority carrier (electron) is injected from the emitter layer 1 to the base layer 3. FIG. 2A and 2B are respectively diagrams schematically showing the bottom energy level of a conduction band of this HET, in which a one-dot chain line indicates Fermi level E.sub.F. FIG. 2A schematically illustrates a state in which no voltage is applied to each terminal. FIG. 2B schematically illustrates a state in which a voltage Vcc is applied between the emitter E and the collector C with the collector C side being positive in polarity. In FIG. 2B, a broken line indicates a state in which no voltage is applied between the emitter E and the base B, and a solid line indicates a state in which the predetermined on-state voltage V.sub.BE is applied between the emitter E and the base B with the base B side being positive in polarity. Under the state that the voltages Vcc and V.sub.BE are respectively applied to the HET, as shown by the solid line in FIG. 2B, a tunneling or thermionic emission is generated by the majority carrier (electron) which respectively tunnels through or overflows the emitter barrier layer 2, and this majority carrier is injected into the base layer 3. In this case, the emitter barrier layer 2 is made to have such a barrier height h as to substantially neglect the thermionic emission current as compared with the tunneling current. At this time, under the state that the on-voltage V.sub.BE is applied across the emitter E - base B path, the electron with large kinetic energy injected into the base layer 3, or a so-called hot electron is directed to run toward the collector layer 5. At this time, a part of the electron is changed in its transport direction and loses its energy due to the scattering in the base layer 3, and is dropped to the bottom of the conduction band of the base layer 3, which then becomes a base current I.sub.B. Other electron reaching to the collector layer 5 over the collector barrier layer 4 becomes a collector current I.sub.C. At this time, the emitter current I.sub.E is expressed as I.sub.E =I.sub.B +I.sub.C and a current gain .beta. is given by the following equation. EQU .beta.=I.sub.C /I.sub.B
Under the state that the voltage V.sub.BE is not applied across the base B - emitter E path, the emitter barrier layer 2 becomes large in thickness as shown by a broken line in FIG. 2A so that the carrier becomes difficult to tunnel from the emitter layer 1 to the base layer 3, thus the number of the injected carrier being reduced. Further, the collector barrier layer 4 becomes high in barrier height h for the injected carrier so that the transport of the carrier toward the collector layer 5 is blocked, thus the collector current I.sub.C being suppressed. As a result, depending on the voltage applied to the base terminal B, the HET is turned on and off similarly to the ordinary transistor operation.
In order to have a large current gain .beta. in this HET arranged as mentioned above, the width (thickness) of the base layer 3 is desired to be made as small as possible so as to obtain as high transfer ratio as possible. If, however, the thickness of the base layer 3 is reduced, the amount of the carrier in the base layer 3 is reduced so that the resistance in the base layer 3 becomes large. Consequently, the base voltage applied to the base layer 3 becomes difficult to be applied to its whole base area and also there arises a defect that the ohmic contact of the output terminal with the base layer 3 will become difficult to be made.
While the mobility of electron in GaAs becomes high in the .GAMMA. band, a scattering probability thereof becomes high in a band of high energy level, namely, X band and L band. FIG. 3 illustrates a scattering probability of GaAs with respect to the electron energy at 300.degree. K. In FIG. 3, a curve 7 indicates a scattering probability of an electron scattered from .GAMMA. band to X band. From the curve 7, it will be clear that when the electron energy exceeds an upper valley, or the levels of the X band and L band, the scattering of the electron from the .GAMMA. band to the X band or from the .GAMMA. band to the L band becomes rapidly large. Since the scattering of this kind changes the direction of the injected electron irregularly, this scattering is very harmful for the transport of the electron, lowering the mobility of the electron. In FIG. 3, a curve 8 indicates a scattering of an optical photon. As will be clear from the curve 8, although a large scattering probability is exhibited in the low energy side, the scattering takes the form of a small angle so that the influence for the electron transport is substantially small. Accordingly, when GaAs is used as the base layer, while the scattering of the electron depends on the thickness of the base layer, in order to efficiently reduce the scattering, the energy of the electron injected from the emitter layer is desired not to exceed the energy of the upper valley (normally in a range from 0.31 to 0.35 eV). To this end, firstly the barrier height h of the collector barrier layer must be selected not higher than 300 meV, which is less than the energy of the upper valley and, for example, 200 meV.
In the transistor operation, however, a breakdown voltage between the base B and collector C becomes a serious factor. More particularly, when in the HET a voltage is applied between the base B and the collector C, if the breakdown voltage between the base B and the collector C is zero or very small, a thermally disturbed electron overflows to the collector layer from the base layer through the collector barrier layer which then allows the collector current to flow regardless of a base potential, thus the HET no longer carries out the so-called transistor operation.
FIG. 5 illustrates a current versus voltage characteristic of a diode with GaAs/AlGaAs/GaAs triple-layer structure as shown in FIG. 4 under the condition that the thickness of the AlGaAs layer used as a barrier layer in the diode is selected to be 500 .ANG.. In FIG. 5, curves 9, 10, 11 and 12 indicate current versus voltage characteristics with the barrier height h of the barrier layer being selected as h=350 meV, h=300 meV, h=250 meV and h=200 meV, respectively. From FIG. 5, it will be seen that if, at 0.5 V, a leakage current thereof is suppressed to be not higher than 1 .ANG./cm.sup.2, the barrier height or the height h of the collector barrier in the transistor must be selected to be not lower than 300 meV. Under low temperature of 77.degree. K., it is necessary to select the height h of the collector barrier as 150 meV.
As described above, with respect to the breakdown voltage between the base B and the collector C, it is desired that the barrier height h of the collector barrier is selected to be at least not lower than 300 meV. If, however, the height h of the collector barrier is merely increased, only the electron of high energy existing in the X band and L band can flow over the collector barrier so that the scattering probability of the electron becomes large and the current gain .beta. is lowered. Accordingly, in the conventional HET, the current gain .beta. is equivalent to the height of about 0.1 and this falls far short of the desired value of the current gain .beta. that the transistor is expected to have, for example, 10 to 100. In addition, due to the above-described factors such as the breakdown voltage and so on, the HET can only be used at low temperature of 77.degree. K. so that this prior art HET can not be expected to operate at room temperature or high temperature near the room temperature.